Connecting to a Windows server allows OS X users to mount their home drive (using SMBFS) and get notifications when passwords are about to expire.
When you are not connected to the network you can still login to your computer, which is of particular use for Powerbooks. Another benefit is that no changes are needed in the Active Directory schema, which will help to keep Windows network administrators happy.
The most useful feature of MSAD Login is the share menu (status bar menu) that can be used to easily mount network shares.